<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>显示当前系统时间</button>
    <div>某个时间</div>
    <script>
        //1.获取元素
        var btn = document.querySelector('button');
        var div = document.querySelector('div');
        //2.注册事件
        btn.onclick = function() {
            div.innerText = getTime();
        }

        function getTime() {
            var date = new Date();
            var hours = date.getHours();
            hours = hours < 10 ? '0' + hours : hours;
            var minutes = date.getMinutes();
            minutes = minutes < 10 ? '0' + minutes : minutes;
            var seconds = date.getSeconds();
            seconds = seconds < 10 ? '0' + seconds : seconds;
            return hours + ":" + minutes + ":" + seconds;
        }
    </script>

</body>

</html>